Laptop in crisis! need help asap?

hi folks i want to know what to do with my dell inspiron e1505 laptop that has been infected with either virus or spyware, now its been crashing all over the place and i cant even login to my account because of extreme lag. i want to know how to reformat this laptop although there's a bit of a problem that follows that. My friend turned my laptop which had originally vista basic as its OP into windows xp pro. i know that vista computer usually have that partition drive for reformatting but now that my laptop is xp i dont have that anymore. please tell me what to do so i can reformat this laptop. it doest matter if it becomes vista again or xp. thank you very much in advance for the answers